The present invention relates to a volume phase grating using Bragg reflection of returning, out of an incident light, only rays of a specified wavelength to an incident-light side by reflecting them, a method for producing such a volume phase grating, and an optical module and a semiconductor laser module using such a volume phase grating.
A Fiber Bragg Grating (abbreviated as FBG) has been known as a means for returning, out of an incident light, only rays of a specified wavelength to an incident-light side in an optical fiber. The FBG is such that refractive index is cyclically changed along longitudinal direction (optic-axis direction) in a fiber core 124 as shown in FIG. 12. This FBG can be produced by projecting a recording light 130 having a wavelength λuv in an ultraviolet range to an optical fiber 110 via a phase mask 136 having a mask interval Λ (mask) and forming phase gratings 120 for causing the refractive index to be cyclically changed along the longitudinal direction in the fiber core 124 by a photoinduced refractive index change for transferring and forming an intensity modulation area onto the optical fiber 110 by the interference of diffracted rays 125 of ± first order from the phase mask 136. A grating interval Λ(FBG) determining a refractive index changing cycle in the FBG satisfies the following relationship:Λ(mask)=2×Λ(FBG).The properties of the thus produced FBG 114 are determined by a change of the refractive index, the grating cyclic interval Λ(FBG) and a length of the FBG 114 along optic-axis direction. The change of the refractive index and the length of the FBG influence a reflectance and a band width, whereas the grating cyclic interval Λ(FBG) influences a center wavelength. A center wavelength λb of the reflection by the FBG 114 where the grating cyclic interval Λ(FBG) is constant with respect to the longitudinal direction of the optical fiber 110 is given by:λb=2×n×Λ(FBG)where n: effective refractive index of the fiber core.
If the FBG 114 is constructed at an output-side end 117 of a semiconductor laser module 116 and is coupled via coupling lenses 117 and a part (about several to 10%) of a light emitted from a semiconductor laser diode 112 is returned to the semiconductor laser diode 112 as shown in FIG. 11, thereby letting the FBG 114 function as an external resonator, an output-wavelength spectrum characteristic of the semiconductor laser diode 112 can be modified to have a narrower range and can be stabilized. Further, an emission spectrum characteristic of the semiconductor laser diode 112 substantially coincides with the reflection center wavelength λb of the FBG 114 as an external resonator. Further, the output-wavelength spectrum characteristic and the output characteristic in relation to a temperature change can be stabilized (see Japanese Unexamined Patent Publication No. H09-283847).
There is also known a volume phase grating in which phase gratings are formed in a SiO2 or a glass material to provide a cyclical change of the refractive index instead of being formed in the FBG 114 forming reflecting diodes in the form of phase gratings in the optical fiber 110. This volume phase grating is called a Volume Bragg Grating, (abbreviated as VBG), particularly in the case of being used such that the diffracting direction of launching angle of the diffracted rays of ± first order coincide with the angular direction of the reflected lights.
FIG. 10A shows a method for forming the volume phase grating. More specifically, recording lights 130 are projected for exposure for about 5 to 30 min. to an outer surface 122 of a phase grating substrate 103 made of an induced refractive index medium (additive such as silver is added to an oxidized glass such as a SiO2 base) having the upper and lower surfaces thereof optically polished and having a thickness D with the lengths of optical paths from unillustrated beam splitters to the outer surface 122 of the phase grating substrate 103 precisely coincided. An exposure time changes depending on the material of the phase grating substrate 103 used. A grating cyclic interval P1 can be arbitrarily set by adjusting an angle θ0 of the recording lights 130 to the phase grating substrate 103.
The recording light 130 to be projected to the substrate 130 is such that a recording light having coherence and a wavelength of λuv (e.g. a light having a wavelength of 458 to 528 nm in the ultraviolet range and emitted from an argon laser) is split halfway into two, for example, by means of a beam splitter and the respective resulting lights are converged once by a lens and made into a parallel light after being passed through a pinhole (diameter: 5 to 25 μm) provided at a focusing position in order to eliminate unnecessary diffracted rays.
If it is assumed that the wavelength of the recording light 130 is λuv and the angle thereof to the substrate 130 is θ0, the following relationship holds in accordance with the Snell law of refraction:n0×sin θ0=n1×sin θ2.
If a refractive index n0 of air is 1, an angle θ2 in the substrate 130 is:θ2=sin−1{sin θ0/n1}where n1: refractive index of the phase grating substrate 103 and n0: refractive index of air (=1).
Further, a wavelength λm in the phase grating substrate 103 having a refractive index of n1 at the wavelength λuv is given as follows because a light velocity Cm in the phase grating substrate 103 is 1/n1 (frequency f is constant) of a light velocity Cuv in the air:Cm=Cuv/n1.From Cm=f×λm, Cuv=f×λuv,λm=λuv/n1.
If two lights of plane waves having an amplitude A at the wavelength λm intersect in the phase grating substrate 103 having the refractive index of n1, the grating cyclic interval P1 of the phase gratings 120 is determined by a place where a combined amplitude of the respective plane waves becomes 0 due to the above interference, and the phase gratings 120 become a group of straight lines defined by:2A×[cos{(2×π×Y×sin θ2)/λm}]=0where A: amplitude of the respective plane waves, Y: position on Y-axis, and Y(k)={(2×k+1)×λm}/(4×sin θ2) (k: arbitrary integer).
Therefore, the grating cyclic interval P1 of the respective phase gratings is given by:P1=Y(k+1)−Y(k)P1=λm/{2×sin θ2}.
Specifically, lines of the specified cycle defined by the above equation are exposed. If the phase grating substrate 103 having the recording lights 130 projected thereto for exposure is left in a high-temperature environment of about 500° C. for several hours, a refractive index changing area where the refractive index cyclically changes appears in the phase grating substrate 103. A change Δn of the refractive index is about 0.01 to 0.001.
Thereafter, the phase grating substrate 103 is vertically so cut between the upper and lower surfaces as to have a width T as shown in FIG. 10B, whereby a plurality of volume phase gratings 137 having a height of D mm and a width of T mm as shown in FIG. 10C can be obtained. AR coatings (anti-reflection coatings) made of dielectric multi-layer films are applied to incident surfaces 121 obtained by optically polishing cut surfaces 126 in order to prevent reflection at the incident surfaces 121.
Incident lights 118 having wavelengths λa, λb and satisfying Bragg condition are caused to fall on the volume phase grating 137 obtained by the above process as shown in FIG. 10C, only the light having the wavelength of λb is reflected and the light having the wavelength of λa undergoes an end-face reflection (indicated by arrows 127).
Although not shown, if the light is obliquely incident at an angle of α, the diffracting direction and the angle of reflection coincide with respect to the formed phase grating surfaces, thereby satisfying the Bragg diffraction condition. The diffraction efficiency depends on the cutting width T (number of the phase gratings 20). If the light having a wavelength different from that of the Bragg diffraction condition is incident or the light is incident at an angle different from the Bragg diffraction condition, the diffraction efficiency is reduced and a launching angle of the diffracted rays changes.
However, the conventional construction example using the phase gratings having the above properties has the following problems. Specifically, if the FBG 114 is constructed at the output-side end 117 of the semiconductor laser diode 112 in the semiconductor laser module 116 as shown in FIG. 11, when lights having wavelengths other than that of the light emitted from the semiconductor laser diode 112, particularly lights having wavelengths near the oscillating wavelength of the semiconductor laser diode 112 directly enter the semiconductor laser diode 112, the oscillating wavelength becomes unstable due to the influence of such lights, thereby making the output-wavelength spectrum characteristic and the output characteristic unstable.
In order to eliminate such an influence, an optical isolator for eliminating such lights may be mounted at an output side of the semiconductor laser diode 112. However, in the case of mounting the optical isolator (not shown), a necessary reflected light 119 from the FBG 114 functioning as an external resonator is cut off before reaching the semiconductor laser diode 112. Thus, the FBG 114 cannot function as an external resonator.
Such unnecessary lights may be removed if an inline type optical isolator (not shown) is mounted at an output side of the FBG 114. However, this results in a higher cost and the semiconductor laser module becomes an assembly of a plurality of modules, thereby necessitating more space for parts. Further, an area of the FBG 114 in the output-side fiber end 117 is normally as long as about 10 mm. Thus, upon a large temperature change, the grating cyclic interval P1 itself changes due to a linear expansion. Therefore, the wavelength of the reflected light 119 changes, which results in a problem that the oscillating wavelength of the semiconductor laser diode 112 is changed.
In the case of the optical module or the semiconductor laser module using the conventional volume phase gratings 137, if the incident light 118 is caused to be perpendicularly incident on the volume phase gratings 137 as shown in FIG. 10C, the end-face reflection 127 occurs, although to a small extent, even when the AR coating is applied to the incident surface 121. Thus, unnecessary lights having the wavelength λa other than the one satisfying the specific Bragg condition are also reflected and returned to the semiconductor laser diode 112, thereby causing an unnecessary oscillation in the semiconductor laser diode 112. This disadvantageously makes the output-wavelength spectrum characteristic unstable.